Priskin was dismissed for an elbow on Darren Purse after John-Joe O'Toole and Will Hoskins gave Watford a 2-0 lead.
Boothroyd said: "It was a crowded corner and there was a little bit of grabbing going on, unseen by the referee, and he's reacted.
"It doesn't matter what happened, you can't react. The best thing to do in that situation is to pop one in the net and then smile.
"He's a young boy and all our players have to understand that indiscipline is not acceptable because it can cost a team."
Queens Park Rangers could only draw 0-0 with Swansea despite the Welsh side playing without a recognised goalkeeper for most of the match.
Dorus de Vries was forced off after breaking his jaw in a collision with Martin Rowlands and, with no substitute goalkeeper on the bench, defender Alan Tate has to go in goal.
Rangers manager Iain Dowie said: "They had an outfield player in goal and we haven't tested him at all that is not good enough."
